The present invention relates to a process for purifying adipic acid by crystallization or recrystallization from at least one carboxylic acid.
Adipic acid is one of the two base materials for preparing polyamide 6-6. For the applications of polyamide 6-6 it is necessary to have a very high purity, and this purity must exist already at the stage of the precursors, especially at the adipic acid stage.
Depending on the process by which adipic acid is prepared the impurities it contains are obviously different. The present process can be applied to adipic acid originating from various processes of synthesis. In fact, one of the most troublesome and sometimes most costly impurities is formed by the presence of traces of the catalyst employed during the preparation of adipic acid.
However, in the description which follows, the process will be applied more particularly to adipic acid obtained from the double hydroxycarbonylation of butadiene or from the oxidation of cyclohexane.
The first hydroxycarbonylation of butadiene leads to a mixture of pentenoic acids, principally 3-pentenoic acid. The second hydroxycarbonylation affects the pentenoic acids obtained in the first reaction and leads to adipic acid which also includes a certain amount of 2-methylglutaric acid, 2-ethylsuccinic acid and other compounds originating from the first hydroxycarbonylation reaction, such as gamma-valerolactone, unconverted pentenoic acids, and methylbutenoic acid. It also includes traces of the catalyst employed in the second hydroxycarbonylation reaction, usually iridium and/or rhodium.
The direct oxidation of cyclohexane to adipic acid is generally carried out in the presence of cobalt, and in this process the adipic acid obtained contains traces of cobalt catalyst.
Since adipic acid is of low solubility in water when cold but is much more soluble when hot, this solvent is generally employed for the crystallization of the said acid.
However, owing to the very high purities which are increasingly required for adipic acid, especially insofar as trace metals are concerned, one or even a number of recrystallizations from water often prove to be inadequate.
Besides the trouble which can be caused by the presence of trace metals to the various uses of adipic acid, the intrinsic value of certain catalysts, such as iridium or rhodium, bearing in mind the very large tonnages of adipic acid, means that it is essential to recover them as thoroughly as possible in the context of an economically viable industrial process.
The present invention consists in an improved process for crystallization or recrystallization of adipic acid, characterized in that the said crystallization or recrystallization is carried out in at least one carboxylic acid having a melting point of less than 20xc2x0 C.
The carboxylic acids employed in the present process are, more particularly, aliphatic carboxylic acids which are saturated or which contain an ethylenic unsaturation.
They are preferably linear or branched monocarboxylic acids having 2 to 6 carbon atoms.
As nonlimitative examples of such monocarboxylic acids mention may be made of acetic acid, propionic acid, butanoic acids, pentanoic acids, hexanoic acids and pentenoic acids.
Acetic acid and pentenoic acids are preferred, acetic acid owing to its availability and to its use in the synthesis of adipic acid from cyclohexane, and pentenoic acids because they are an intermediate in the preparation of adipic acid from butadiene.
The purity of the adipic acid recrystallized in this way may be improved further when recrystallization is carried out in the presence of carbon monoxide.
The carbon monoxide can make up at least part of the atmosphere above the solution in the crystallization or recrystallization reactor (or reactor headspace) or can create within the said reactor a pressure which is greater than the atmospheric pressure.
In practice, the process will therefore be operated under an absolute pressure of from 0 bar (preferably at least 0.5 bar) to 50 bars of carbon monoxide, the upper limit not being critical in nature but being representative of industrial apparatus which is not excessively expensive.
The crude adipic acid subjected to recrystallization according to the present process is usually an adipic acid which has already undergone one or more purification treatments, in particular by crystallization from water, by refining or else by distillation, to give it a minimum purity of approximately 95%.
Generally, the adipic acid recrystallized by the process of the invention has a purity of from 95 to 99.95%.
The recrystallization consists in taking the adipic acid to be purified and dissolving it in the minimum amount of hot aliphatic carboxylic acid, i.e. usually at a temperature from 80 to 250xc2x0 C., optionally under an at least partial pressure or atmosphere of carbon monoxide, and in then inducing crystallization of the dissolved adipic acid by cooling the solution, optionally after having seeded the solution using crystals of pure adipic acid.
Generally, the quantity of carboxylic acid employed is that which leads to a saturated solution of adipic acid at the chosen temperature. By way of indication, at 90xc2x0 C. the saturated solution in 3-pentenoic acid contains approximately 33% of adipic acid by weight per weight.
The catalyst content of the adipic acid can also be reduced when recrystallization is carried out in the presence of a strong protic acid.
By strong protic acid is meant in the present text an inorganic protic acid having a pKa of less than 1.
As nonlimitative examples of such strong protic acids mention may be made of hydroiodic acid, hydrobromic acid, hydrochloric acid, nitric acid and sulphuric acid.
The quantity of strong protic acid can vary from 0 mol to 100 mol per mole of catalyst metal present in the adipic acid. Preferably, the quantity of protic acid varies from 0 mol (more preferably from 1 mol) to 50 mol per mole of catalyst metal.
The process of the invention likewise embraces the crystallization of adipic acid from reaction mixtures in which it is present.
It is thus possible, for example, to crystallize adipic acid from the mixture obtained by hydroxycarbonylation of pentenoic acid with water and carbon monoxide. This reaction mixture can be mixed with the carboxylic acid in the presence or absence of the carbon monoxide employed for the hydroxycarbonylation reaction, and the whole mixture can be kept at a temperature from 80 to 250xc2x0 C. as indicated above for the recrystallization.
Crystallization can also be carried out in the hydroxycarbonylation reactor by allowing the reaction mixture to cool, preferably under carbon monoxide pressure. This variant can be employed in particular when hydroxycarbonylation is conducted in a carboxylic acid or when it is conducted in 3-pentenoic acid with a degree of conversion of the latter which is incomplete.
Since the hydroxycarbonylation reaction is conducted in the presence of carbon monoxide, it is not generally necessary to add this compound for the crystallization, although this possibility is not excluded if appropriate.
Similarly, since the promoter used in the hydroxycarbonylation reaction may be hydroiodic acid or hydrobromic acid, it may not be necessary to add the strong protic acid. However, if desired, the quantity of strong protic acid present in the reaction mixture may be supplemented. As for the recrystallization of adipic acid, it is also possible to operate in the absence of strong protic acid, although this variant is not preferred.
The recrystallization according to the invention can be carried out a number of times in succession on the adipic acid in order to reduce further the content of catalyst metal. It is also possible to follow a crystallization or recrystallization according to the invention by one or more recrystallizations from water.